Judging from the content of the message you give as an example, the sender of the message could be assuming you're someone else. It's not unusual for women to give fake numbers to strangers who flirt with them if they're not interested in actually staying in contact. Maybe someone gave your number away by pure chance.
In that case, you should be very straightforward to avoid any confusion.
I don't recognize this number. Could you be confusing me with someone?
I think someone gave you the wrong number.
If you think the sender might actually know you, it's better to ask for their name.
Sorry, but I don't recognize this number. Please tell me who you are.
What you should not do is ignore the message. Although it's not uncommon to be ignored after a flirt, the other person could actually be someone you know or could try to contact someone using an outdated number. Not replying usually leads to receiving more messages and would frustrate the texter if they're trying to contact someone else.
Something comparable happened in my family: my mother and I got new phone numbers at the same time and were texted by strangers shortly after. I informed them that the person they try to contact has a new number now and stopped receiving messages very soon. My mom ignored these messages and still complaints about "strange people texting her and not realizing that she's not interested". Well... how should they realize?